1. Field of the Invention
The present invention relates to a pyrolysis process for treating sewage sludge, more particularly a pyrolysis process for sewage sludge which reduces the formation of harmful hexavalent chromium even if the sewage contains chromium.
2. Description of the Prior Art
The sewage sludge formed by treating sewage is generally dehydrated by adding a dehydrating agent, such as ferric chloride or calcium hydroxide and then incinerated at a temperature of 800.degree.-900.degree. C. under an oxidizing atmosphere and finally discharged as an incinerated ash for reclamation. However, when the sewage sludge contains chromium, trivalent chromium in the sludge is oxidized into harmful water soluble hexavalent chromium in the course of incinerating, so that if such an incinerated ash is discharged from reclamation, the harmful hexavalent chromium in the incinerated ash is released into underground water, rivers and the like by rain water, which constitutes a risk of environmental pollution. Therefore, in the incinerated ash containing hexavalent chromium, a post-treatment for preventing harm of hexavalent chromium is necessary. For this purpose, the incinerated ash is solidified into concrete by using cement, the hexavalent chromium in the incinerated ash is reduced by adding a reducing agent or the incinerated ash is fused and solidified at a high temperature of 1,300.degree.-1,500.degree. C., but such processes are complicated in the treating step and there is fear that secondary pollution is caused and furthermore these processes become commercially expensive.
An internal heating type or an external heating type of pyrolysis process, in which thermally decomposed gas of sewage sludge is not burned in a pyrolysis furnace, has been recently proposed. In these pyrolysis processes, the formation of hexavalent chromium can be reduced but the sewage sludge is treated only by the thermal decomposition, so that the treating capacity per unit time is small and a large amount of fuel for generating hot gas is needed and further tar contained in the exhaust gas discharged from the pyrolysis furnace deposits on pipes and instruments and trouble often occurs in view of the operation.